Epub does sound like your best option, although reflowing text around an image on a 6" screen may be overkill. Sony has said they will support Adobe's Digital Editions, but have not given a time line or any other details (it isn't available for Linux yet anyway). Teleread has a post on the state of epub support:
New IDPF format: Which software companies are aboard—and how .epub will help publishers. It may be a bit optimistic, but note the lack of e-ink device vendors (iRex is probably via MobiPocket). Also, MobiPocket may "support" .epub by converting it to .mobi and I think that rules out CSS support.